AbstractThe effects of triphenyl phosphite (TPP) on the radical polymerization of styrene (St) and methyl methacrylate (MMA) initiated with α,α,‐azobisisobutyronitrile (AIBN) was investigated at 50°C. The rate of polymerization of St and MMA at a constant concentration of TPP was found to be proportional to the monomer concentration and the square root of the initiator concentration. The rate of polymerization and the degree of polymerization of both St and MMA increased with increasing TPP concentration. The accelerating effect was shown to be due to the decrease of the termination rate constantktwith an increase in the viscosity of the polymerization systems. The chain transfer constantCtrof TPP in St and MMA systems was determined from the degree of polymerization system. TheCtrof TPP was almost zero in the St system and 6.5 × 10−5in the MMA